1. Field of the Invention
The present invention relates to a device, intended for use in an anaesthetic administration apparatus, for identifying at least one liquid anaesthetic in the anaesthetic administration apparatus.
2. Description of the Prior Art
In general, narcosis or anaesthesia involves inducing a patient into a dormant state in which he or she is incapable of feeling any pain. The patient is usually administered a mixture of oxygen, nitrous oxide, an anaesthetic agent and (possibly) air from a respiratory circuit in an anaesthetic apparatus. The most common anaesthetic agents are halothane, desflurane, enflurane, isoflurane and sevoflurane. The anaesthetic agent is normally in liquid form in an anaesthetic vaporizer attached to the anaesthetic apparatus, and a desired amount of liquid anaesthetic is vaporized and sent to the respiratory circuit during anaesthesia.
The effect of different anaesthetic agents on the patient, which are administered to the patient in varying concentrations, varies. The side-effects of the different anaesthetic agents also vary. An anaesthetic apparatus is available which can be equipped with a number of anaesthetic vaporizers so as to give the anaesthesiologist a direct option of selecting the anaesthetic agent he or she deems best for the patient, with no need to mount or detach different anaesthetic vaporizers. In addition, the anaesthesiologist is able to switch to a different anaesthetic agent during ongoing surgery. This occurs in operations on e.g. infants or in protracted surgery.
The patient should never be given a mixture of different anaesthetic agents, since the effect of the mixture is hard to predict and largely unknown. That is why anaesthetic apparatuses are normally devised so they are only able to administer one anaesthetic agent at a time to the patient. The possibility of mixing a number of anaesthetic gases, however, is still a risk, even with an anaesthetic apparatus with only one anaesthetic vaporizer, viz. if different anaesthetic agents became intermixed when replenished anaesthetic agent is added to the active anaesthetic vaporizer.
To minimize the risk of errors in inducing anaesthesia, it would be advantageous if the anaesthetic system could automatically identify the anaesthetic agent administered to the patient. The anaesthetic apparatus then could be devised to issue an alarm if an error occurred.
As noted above, the various known anaesthetic agents are administered to the patient in different concentrations. If an erroneous concentration is set for a particular anaesthetic agent, the patient could be subjected to needless risk, entailing both overdosing and underdosing of the anaesthetic agent. An erroneous concentration will therefore most likely occur when the wrong anaesthetic agent is administered, since different concentrations are used for the different anaesthetic agents.
Even these risks would be greatly reduced if the anaesthetic agent could be identified before it is supplied to the patient.
In the anaesthesia field, identification of anaesthetic agents by optical methods, i.e. by absorption spectrophotometry, is known. Since a number of anaesthetic agents have similar absorption spectra, absorption must be measured at a number of wavelengths to permit reliable identification of a specific anaesthetic agent. This makes it necessary to use extensive measurement equipment for identifying an anaesthetic agent. Optical measurement is generally performed on the gas mixture supplied to the patient, i.e. the anaesthetic agent is present in gaseous form.
Another known way of identifying anaesthetics in gaseous form to employ two different measurement methods for determining the concentration of the anaesthetic agent, and to combine the results of these two methods to identify the anaesthetic agent. For example, an optical method, in which measurement of absorption is made at a specific wavelength for the anaesthetic agent, and a method using an oscillating crystal, coated with a layer of oil or grease which adsorbs and desorbs anaesthetic gas molecules, can be used. The crystal""s oscillation frequency changes to varying degrees depending on the molecular weight and concentration of the anaesthetic gas. When a combination of these two measurement methods is employed, a unique signal is obtained for every known anaesthetic agent. It should be noted that measurement of changes in the oscillation frequency of a crystal is not in itself sufficient to permit identification of anaesthetic agents, since the frequency change is relatively the same for the different anaesthetic agents in the concentrations at which they are normally used. One such device is described in U.S. Pat. No. 5,272,907.
An object of the present invention is to provide a device, intended for use in an anaesthetic apparatus, for reliably and simply identifying at least one liquid anaesthetic in the anaesthetic apparatus. The device according to the present invention has a measuring unit for determining at least one parameter related to the refractive index of the liquid anaesthetic and a unit for determining the temperature of the liquid anaesthetic. The device further has an analysis unit for identifying the anaesthetic agent from the determined parameter.
One advantage of the device according to the present invention is that the analysis unit is able to determine, from the established parameters, whether a mixture of at least two anaesthetic agents is present. This further enhances patient safety in anaesthesia. Preferably, the analysis unit also identifies the liquid anaesthetics that have been mixed and the concentration of each liquid anaesthetic.
A first possibility to obtain the aforementioned parameter in accordance with the invention is to employ a light-emitter arranged so a light beam emitted thereby strikes the surface of the liquid anaesthetic at a specific angle of incidence. The device according to the present invention also has a position detector which is arranged to indicate any deflection of the light beam in the liquid. This deflection constitutes the parameter and depends on the liquid anaesthetic""s refractive index and level.
The level can be standardized (maintained constant) by selecting the appropriate arrangement, e.g., by using a small container which receives a precise amount of liquid from the vaporizer. Such a test can be made at the onset of use of the anaesthetic apparatus and each time the vaporizer is filled with liquid anaesthetic. When the level is constant, the identity of the liquid anaesthetic can be determined based on temperature and deflection (refractive index).
In conjunction herewith, it is advantageous to arrange a measuring rod on the position detector to indicate the identity of the anaesthetic agent when the light beam strikes the position detector. If the light beam shines between two different xe2x80x9cidentityxe2x80x9d positions, this means there is a mixture of different anaesthetics.
A second possibility to obtain the aforementioned parameter is for the measuring unit also to have an optical fiber which has one end adjacent the light-emitter means and another end arranged adjacent a photosensitive cell and a curved section immersed in the liquid anaesthetic. The measuring unit determines the amount of light which leaks out of the curved section of the optical fiber as the parameter.
Although similar devices are known (see e.g., U.S. Pat. No. 4,187,025) for determining refractive index or temperature of a known liquid, or discrete levels of a liquid in a container, they have not been utilized, nor suggested for use in determining the identity of anaesthetics in anaesthetic machines.
According to an embodiment of the invention, the part of the optical fiber immersed in the liquid anaesthetic has straight sections in addition to the curved section. In particular, the straight sections pass through the surface of the liquid anaesthetic at a right angle. This results in minimum light loss from the fiber is obtained as it passes from one surrounding refractive index (e.g., air) to another (the liquid anaesthetic).
The measuring unit can also have a second optical fiber, not immersed in the liquid anaesthetic, of the same length as the first fiber. This second fiber is intended to supply a reference value.
In a second embodiment of the invention, the measuring unit also has n similar optical fibers arranged at different levels in the liquid in order to determine the level of the liquid in discrete steps.
At least one of these optical fibers has a number of curved sections at different levels in the liquid in order to determine the level of the liquid.
The analysis unit can issue an alarm when the level of the liquid drops below the curved section of the fiber.
In a third embodiment simultaneous level measurements are made with the parameter determination in accordance with the invention wherein the device also has two electrodes for contacting any liquid anaesthetic which is present and for measuring the capacitance of the liquid between the electrodes. Since the liquid""s capacitance varies with the level of the liquid, measuring this capacitance constitutes a level measurement.
In a first version of this third embodiment, the immersed part of the optical fiber has two substantially parallel sections, and the two electrode means are arranged along these parallel sections.
The two electrodes can be formed by respective metallized surface areas on the parallel sections of the optical fiber.
Alternatively the two electrodes can be formed by metal wire wound around the respective parallel sections of the optical fiber.
In a second version of the third embodiment, a first of the electrodes is formed by a metallized surface area on the optical fiber, and a second of the electrodes is formed by a metallic deposit on the inside of a container for the liquid anaesthetic.